Twilight vision (mesopic vision) is the term for a combination between photopic vision and scotopic vision in low but not quite dark lighting situations. This range comprises ambient brightness from approximately 32 to 0.0032 candlepower per square meter (cd/m2). In traffic, this range of brightness occurs at dusk and at night in the headlights of a motor vehicle.
In twilight vision, visual perception is not as good as in daylight vision. The primary difference is an increase in the threshold of perceivable contrast. The principal causes are higher-order visual defects (aberrations) which have an adverse effect on the ability to see contrast as the diameter of the pupil increases as the ambient brightness decreases.
Mesopic contrast visual acuity is one of the critical visual abilities, especially in traffic. In contrast to daylight visual acuity, which is tested under high contrast conditions, at night, larger structures with lower contrast play the more important role. The measurement of mesopic contrast acuity is a complex determination of visual acuity and sensitivity to contrast, during which only the contrast is varied. The contrast threshold identified is called “twilight visual acuity”.
The methods for the determination of mesopic contrast visual acuity with and without glare were originally developed to check the visual acuity that a driver must have for night driving. However, the examinations also yield diagnostic information on the transparency of the refractive media on one hand and the neuronal visual functions on the other hand (diabetic retinopathy, AMD, glaucoma) and are currently of major importance for a comprehensive evaluation of the quality of vision and after refractive surgical interventions.
On the appliances of the prior art for the examination of the mesopic contrast visual acuity (twilight vision) and glare sensitivity such as contrastometers (manufactured by BKG Medizintechnik, Bayreuth), Mesotest II (manufactured by Oculus Optikgeräte, Wetzlar) and Nyktometer 500 (manufactured by Rodenstock Instrumente, Ottobrunn), with defined ambient lighting (for photopic vision at 85 cd/m2, for contrast vision sensitivity 0.032 cd/m2, for glare sensitivity 0.1 cd/m2) test patterns with different contrast conditions are offered which the test subject must recognize. The visual acuity at the respective ambient brightness is determined from the smallest size the patient can still recognize.
In addition, the influences on mesopic visual function are many and range from lighting conditions to optics to the quality of the ocular media and health of the retina, as well as by the optical properties of the eye, such as pupil size.
Understanding the relationship between pupil size and visual function is crucial to surgeons' ability to manage patients' expectations preoperatively, such as during laser in-situ keratomileusis (LASIK) treatment. Generally, large pupils worsen mesopic vision if significant aberrations, especially spherical aberration, are present. Minimizing the amount of induced spherical aberrations can improve mesopic function. Second, glare and mesopic visual complaints increase when a patient has residual refractive error, regardless of pupil size. Larger pupils aggravate the problem, however. For more than a century, ophthalmologists have been aware that defocus depends on pupil size and the location of aberrations on the cornea. When pupils dilate in dim lighting, aberrations induced by refractive surgery that are located in the midperiphery are able to enter the visual system and can worsen visual function.
Therefore, it is important to accurately test mesopic vision prior to LASIK treatments or any other type of treatment in order to better prepare the surgeon for the appropriate treatment approach, as well as to counsel the patient on possible outcomes and effects of the treatment.
In addition, it would be advantageous to combine such testing, and the data obtained therefrom, with the use of adaptive optics. Originally developed to perfect images taken from telescopes, adaptive optics can provide extremely detailed data for use in cornea sculpting.
Adaptive optics, in general, uses wavefront sensing. A wavefront can be thought of as a group of waves traveling through space, so that their combined “front” is a surface. In a planar wavefront, the waves travel in the same direction at the same rate. Images represented with a planar wavefront can be moved from one place to another. If the waves do not fly straight and true to each other, the image represented will not look the same upon arrival. As images come through the atmosphere and into optical elements, such as high-powered telescopes, planar wavefronts are slightly deformed. To compensate for this, sensitive detection systems were devised to quantify wavefront deformation. Astronomers coupled these systems with tunable mirrors, thereby introducing opposing wavefront deformations. These wavefront-sensing components allow the system to adapt to wavefront deformations.
In examining the eye, flat wavefronts from laser sources are sent through the cornea toward the retina. Some of the light bounces off the retina and exits the eye. An array of lenses collects the reflected light so that a sensitive detector can determine the directionality of the beams comprising the wavefront. The data relayed by the detector is quantified and enables a doctor or surgeon to develop an appropriate approach for treatment, such as by corrective lenses or surgery.